1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or printer adopting an electrophotography method or electrostatic storage method and, more particularly, to an operation of controlling to detect the tint and position of the developer image of each color formed in the image forming apparatus.
2. Description of the Related Art
In a color image forming apparatus including a plurality of photosensitive members, misregistration between the images of respective colors occurs due to mechanical mounting errors between the respective photosensitive members, optical path length errors between laser beams for the respective colors, optical path changes of the laser beams, and the like. Furthermore, the image density of each color changes depending on various conditions such as the use environment of the apparatus and the number of print sheets, thereby changing the color balance, that is, the tint.
To solve this problem, the image forming apparatus performs registration correction and density correction between the images of the respective colors. Japanese Patent Laid-Open No. 11-143171 proposes a technique of forming a registration correction pattern and a density correction pattern on an intermediate transfer belt, thereby performing registration and density detection and correction. The technique described in Japanese Patent Laid-Open No. 11-143171 avoids increase in the cost and size of the apparatus by using a single detection unit to detect registration and density correction patterns.
Japanese Patent Laid-Open No. 2006-284892 discloses an arrangement in which during density correction a reference pattern of each color is formed on an intermediate transfer belt in order to use a detection result obtained at a position within a density correction pattern, where an output is stable. More specifically, the position of a density correction pattern to be used for density detection for each color is decided based on the reference pattern. Japanese Patent Laid-Open No. 2001-166553 proposes a technique of forming both registration and density correction patterns on an intermediate transfer belt, and performing registration and density correction operations in one sequence, thereby shortening a time taken for the correction operations.
When registration and density correction operations are performed in a single sequence, a plurality of registration correction patterns and a plurality of density correction patterns may be repeatedly formed on the intermediate transfer belt. This is done to avoid the influence of periodic variations occurring in the rotation period of photosensitive members and rollers for driving the intermediate transfer belt, which are caused by eccentricity of the photosensitive members and rollers. To perform registration and density correction operations in a single sequence, it is necessary to arrange the correction patterns within one round of the intermediate transfer belt. Along with the recent decrease in size of the image forming apparatus, the perimeter length of the intermediate transfer belt has shortened, and thus it is required to perform registration correction and density correction with high accuracy even using patterns with shorter lengths.